The invention relates to a method for determining a partial area of a map described by features, which partial area describes the remaining travel range of a motor vehicle, and to an associated motor vehicle.
Motor vehicles having energy storage systems for their drives have, on the one hand, a remaining travel range dependent on the energy currently stored in the energy storage system and have, on the other hand, a consumption when they cover a certain distance. Therefore, in particular for motor vehicles having navigation systems, methods for determining the remaining travel range of a motor vehicle and informing a driver of this with the aid of the navigation system have been proposed. For example, a travel range can be displayed in a superimposed manner using an area of a map display of a navigation system, which area is determined from maximum reachable points. In this case, regions which can still be reached with the currently stored energy and can be reached without returning can be indicated in one color, and other regions which can be reached by returning to the current location can be indicated in another color (pictorial presentation of a “fried egg” form in which “yellow=return” and “white=reachable”). Such a procedure is known, for example, from DE 10 2008 037 262 A1.
In order to determine the remaining travel range of a motor vehicle, it has been proposed to use algorithms which are also used to determine distances to be traveled to a destination in a navigation system. In this case, the so-called Dijkstra algorithm or a derivative of the Dijkstra algorithm is usually used. The Dijkstra algorithm is based on ultimately moving through the map data from branch possibility to branch possibility starting from the starting position or current position, with an optimum route to a destination being able to be determined using a cost function, often the fastest passage time. Dijkstra algorithms which ultimately feel their way through the map material or expand from a starting position are also suitable for calculating the remaining travel range if the cost function is based on the energy consumed for the section as costs. However, when determining the remaining travel range, there is no special destination since it is necessary to proceed in all directions and the process is aborted if the energy which is consumed along the route corresponds to or exceeds the energy currently stored in the energy storage system. This means that an enormous computation time and a very large amount of calculation complexity are required in order to determine a remaining travel range or points which can currently still be reached.
In addition, the Dijkstra algorithm is very inaccurate for calculating the remaining travel range, at least in the case of larger remaining travel ranges, since the actual consumption additionally depends on the driving style, the ambient conditions, the traffic and the like. The inaccuracy of the Dijkstra algorithm ultimately increases at least quadratically with the energy which is still available, with the result that a relatively large degree of uncertainty can be expected.
It has been proposed to take into account only larger roads, for example freeways and possibly also federal highways, within the scope of the Dijkstra algorithm, but only points where this type of road is also present are then obtained as the maximum reachable points. Roads of a lower category which lead away from these freeways and could potentially be reached are then possibly displayed as unreachable in the display.
An alternative algorithm is proposed in the post-published patent application DE 10 2010 051 546.9-53, in which, instead of the map data of the Dijkstra algorithm, it is proposed to base the latter on area segments which are each assigned energy costs, possibly in a direction-dependent manner, which are needed to cross the respective area segment. Such area segments may be selected, for example, to be relatively large, for instance with a size of 2 km*2 km. “Following” the area segments therefore results in total in a consumption of the motor vehicle on the route followed. In this case, the calculation can be carried out in a similar manner to the Dijkstra algorithm, with an expansion ultimately being made from a first area segment, in which the motor vehicle is currently located for example, to a second area segment which adjoins the first area segment and is assigned the lowest energy costs of all area segments adjoining the first area segment. In the next step, an expansion is then made from the first and second area segments to a third area segment which adjoins the first area segment and is assigned the lowest energy costs, after the second area segment, of all area segments adjoining the first area segment or adjoins the second area segment if the first, second and third area segments are in total assigned lower energy costs than are assigned in total to the first area segment and to the area segment which is assigned the lowest energy costs, after the second area segment, of all area segments adjoining the first area segment. This method is iteratively continued for the next area segments. Alternatively or additionally, the area segments may be expanded in a star-shaped manner and/or recursively in order to calculate the travel range. The travel range calculation on an expansion path can be aborted if the sum of the energy costs of the area segments reaches or exceeds a predefined amount of energy, in particular the amount of energy currently stored in the energy storage system. This algorithm can be carried out with a considerably smaller amount of calculation complexity than the Dijkstra algorithm based on the road system, with the result that resources and time can be saved. Although the algorithm is more heuristic than the Dijkstra algorithm, this is less relevant, in particular in the case of large travel ranges, since the influences which are not detected by the algorithm and have already been discussed above give rise to a certain inaccuracy anyway.
The result of the known algorithms is ultimately a point cloud of maximum still reachable points or area segments which are stored if the residual energy which is still available has been consumed. The points in this point cloud are then associated with an area, for example by a polyline, and it is assumed that the resulting partial area of the map section indicates the reachable regions. However, considerable errors occur in the resultant partial area with this procedure. In experiments carried out by the applicant, errors of up to 30-40%, even more in special cases, occurred, which means that 30-40% of the remaining travel range displayed can in actual fact no longer be reached or 30-40% of the regions displayed as unreachable can be reached. These effects occur, in particular, when certain regions are not developed in terms of traffic. If situated, for example, at the corner of an island, it may be the case that there are no determined reachable end points in certain regions around the current position of the motor vehicle since only the sea simply exists there, but no roads. If short roads end at the coast, for example, without the residual energy being equal to zero, no end points are set at these locations. These roads are therefore displayed as potentially unreachable (which is incorrect). If the end points are now used to calculate the partial area, for example as the result of an envelope calculation, for example a convex envelope, around the Dijkstra end points, actually reachable regions are excluded. However, other effects also result in errors.